KDE SC 4.3.4's codename "Cold" refers to the feature freeze, KDE's development tree is currently in. This Thursday will see the first beta of KDE SC 4.4, bringing significant improvements to the KDE development platform, the Plasma desktop and the individual applications shipped with KDE SC 4.4.
The current release is one of those monthly releases the KDE team ships to provide the users with bugfixes to the stable series. KDE SC 4.3.4 will most likely be the last release of the 4.3 series as KDE has now entered the stabilization period for 4.4, which is to be released in early February 2010. All hands are needed on deck to make KDE SC 4.4 a stable successor to the 4.3 series. Head over to the official announcement or the (probably incomplete) changelog for further information.